Fused quartz glasses, typically made by fusion of refined natural quartz crystal powders, are widely used in the optical, optical fibre and semiconductor industries for their optical properties, chemical resistance, thermal stability or other properties.
In some critical applications, the impurities or inhomogeneities resulting from the use of a natural crystal feedstock may be unacceptable, and then it becomes necessary to substitute a synthetically produced material. Sometimes this may be a synthetically produced powder, but more generally the nighest grade vitreous silica products are made by vapour deposition. Thus a vaporised precursor compound of silicon is fed to a synthesis flame where it is oxidised or hydrolysed to form a stream of silica fume or a flow of micro-particles of silica which is caused to deposit either as a porous silica soot body, which may be dehydrated or doped by heating in a suitable atmosphere, and then sintered to pore-free glass, or alternatively by deposition at such a temperature that the silica deposit sinters directly to a transparent glass.
The latter process, often called the direct deposition process, yields glass of relatively high OH (hydroxyl content), typically 800-1200 ppM by weight, but this is acceptable for many applications, for optical components such as prisms, lenses etc., For larger articles such as windows of furnaces or spacecraft etc., for mirrors, and for the manufacture of photomasks, i.e., the plates which carry the images to be imprinted by a photolithographic process on a silicon wafer, during the manufacture of microcircuits.
The direct deposition process may be operated in either of two modes. In the first of these, shown in FIG. 1 of the accompanying drawings, a burner 11 provides a synthesis flame 12, typically an oxy-hydrogen flame, and is fed, via a central duct 11a with a stream of precursor material. The precursor material can be one or more gaseous chlorosilanes, (e.g. silicon tetrachloride), but more recently chlorine-free precursors have been finding favour. The silicon compound is oxidised or hydrolysed to form a stream of silica fume or a flow of micro-particles of silica which is directed on a substrate forming the domed end 13a of a rotating cylindrical ingot 13, supported within a furnace structure 14. A substantial proportion of the silica generated in the flame deposits on the substrate which is slowly withdrawn (in direction Z) from the furnace, preferably maintaining a substantially constant burner-to-substrate distance. The silica is deposited on the substrate at such a temperature that it sinters directly to transparent, pore-free glass. The ingot may be rotating about a horizontal, vertical or other axis, and may be subjected to oscillatory movement on either or both axes (X and/or Y) perpendicular to that of rotation to spread the thermal load on the ingot end 13a, and thus to increase the homogeneity of the glass deposited or to control the cross-sectional shape of the ingot.
A second geometrical arrangement used for collecting glass by direct deposition is shown in FIG. 2 of the accompanying drawings. This employs a rotating shallow refractory crucible 21, typically lined with zircon or zirconia-based refractory bricks, mounted on a turntable 22. The bottom of the shallow crucible is typically lined initially with a layer of high purity quartz or quartz glass powder 23, or alternatively crushed synthetic vitreous silica glass for maximum purity. Over this crucible is mounted a refractory roof 24 which carries one or more synthesis burners 25. The crucible may be between 1 and 2 meters in diameter and under these circumstances a significant number of burners may be employed. These serve both to heat the crucible to a temperature above the melting point of silica, and also as generators of synthesis flames 26, each of which deposits a stream of silica fume or soot on the surface of a molten glass pool 27 which is generated in the crucible. After an appropriate thickness of glass has been so generated, the crucible is allowed to cool, the refractory walls are removed, and the disk-shaped ingot of glass is taken away to be cut, machined or otherwise formed into the required shape.
The process of FIG. 1 can be used to generate a cylindrical (e.g. circular cylindrical) ingot. This may be of a size suitable for conversion into cylindrical sections if required, e.g., for lenses or mirror blanks, or may be converted by further thermal processes to rod or tube products. However for some applications a cylindrical shape may be an unsuitable starting material. Thus for certain applications where a series of square or rectangular products is required, for example for photomask substrates, these are either machined from an oversize ingot, with evident wastage, or alternatively the cylindrical ingot is re-shaped, for example by heating to softening temperature within a graphite mould of appropriate internal dimensions and, by slumping under its own weight, or by application of pressure, to force the softened silica to take up the shape of the mould. After cooling, the re-shaped ingot may be cut into slices of the desired dimensions. This secondary operation is costly and results in material losses.
Where such shapes are to be cut from one of the large disk-shaped ingots generated by the crucible process of FIG. 2, this involves extensive cutting operations, and again much wastage. It may also be necessary to reject material of unsatisfactory quality, due, for example, to contamination from the refractories of the furnace roof, or from the crucible itself.
Thus for certain shapes of product, notably those of square cross-section, neither of the two major manufacturing methods yields an ingot which may be used directly, and with high materials efficiency. Furthermore neither permits continuous operation as would be desirable for more economical operation, since both are essentially batch processes.
There is thus a requirement for a direct deposition process for synthetic vitreous silica glass, which can be operated continuously and which will generate an ingot of predetermined cross-sectional dimensions, i.e., round, square, rectangular, or other.
This invention seeks to meet that requirement by providing an improved method of forming a shaped body of synthetic vitreous silica glass and an improved furnace For the manufacture of such a shaped body.
According to one aspect of the invention a method of forming a shaped body of synthetic vitreous silica glass includes the steps of generating a mass of molten synthetic silica in a refractory container, part of the boundary of which defines a shaping orifice, and removing the generated synthetic silica from the container through the orifice as a shaped ingot.
The refractory container (e.g. a crucible) is desirably contained within a refractory furnace enclosure. The silica within the container may be kept above sintering temperature by one or more burners, which may conveniently be supported by the roof of the furnace enclosure so that the flame of the or each burner is directly downwardly towards the crucible. Preferably, the synthetic silica is produced by vapour deposition, in which case at least one of the burners should be a synthesis burner. Alternatively, pre-synthesised silica may be supplied to the crucible, for example in the form of powder, crystal or amorphous grain.
Conveniently the shaping orifice is located at the lowest part of the mass in the crucible and the removal involves positively withdrawing the ingot from below, preferably at a rate substantially similar to that at which synthetic silica is being added to the mass.
Preferably, the burner(s) serve(s) both to generate the synthetic silica in particulate form and to heat the melt so that the silica sinters directly to glass in the mass. Optionally additional heat may be imparted by further heating means.
According to a further aspect, the invention provides a furnace for the manufacture of a synthetic vitreous silica ingot, the furnace comprising: a furnace enclosure housing a refractory container, the refractory container being adapted to hold a melt of synthetic vitreous silica; one or more burners extending into the furnace enclosure and adapted in operation to maintain vitreous silica within said container at or above its sintering temperature; a die disposed within a wall of said container, the die including an orifice through which the glass ingot is extruded; and an arrangement of moveable clamps downstream of the orifice, adapted to support the extruded ingot.
Preferably, at least one burner is a synthesis burner, adapted both to deposit synthetic vitreous silica into the refractory container (e.g. crucible) and to assist in maintaining such silica above its sintering temperature. In such an arrangement, the apparatus also includes means to feed oxygen, fuel and silicon-containing precursor material to the or each synthesis burner.
Optionally, the crucible with its die, the ingot and the arrangement of clamps may be rotated synchronously to provide a deposited glass of improved homogeneity.
Again optionally, the crucible with its die, the ingot and the arrangement of clamps may be moved to and fro horizontally in an x-direction, or alternatively in orthogonally disposed x- and y-directions, to permit spreading of the pattern of deposited glass from the one or more burners.
Alternatively a spreading of the pattern of deposited silica can be achieved by like movement of the burner array and the furnace enclosure.